The so-called "metallic enamels" are enjoying rising popularity, particularly in industrial coating, especially in the automobile industry. In the preparation of metallic enamels in most cases aluminum powders of different fineness and surface treatment are used with the metal particles always having a flaky structure in order to achieve the desired effect. This tendency to use "metallic enamels" is counteracted due to the polluting effect of such paints because of the substantially higher level of solvents needed for such paints as compared to paints which do not have this metallic effect.
Further, a difficulty connected with the use of nonprecious metal pigments in aqueous paints is that the fine metal particles react with water with the formation of hydrogen. On the one hand, through this reaction, the desired optical effect is either reduced or destroyed; and, on the other hand, the developing hydrogen gas can lead to the explosion of the paint containers.
Many attempts have been made to overcome the difficulties surrounding the use of aluminum pigments in an aqueous paint. For example, the producers of pigments offer special aluminum pigment pastes. Alcoa Pigments Technical Data, Report No. 100, July 1975, discloses in an article entitled "Aluminum Hydro-Pastes Promise Industry-Wide Impact" a new technology where the aluminum pigment, in addition to a wetting agent, contains a volatile corrosion inhibitor. Similar methods of treatment with water-repellent substances are recommended by others. U.S. Pat. No. 3,926,874 uses polyamides, fatty amides, or fluoro and silicone containing wetting agents with specific surface tensions. According to U.S. Pat. No. 3,839,254, perfluoroalkyl wetting agents are used; while U.S. Pat. No. 4,138,270 uses fatty acids or fatty acid alkanol amides in combination with nonionic wetting agents. A similar method is described in EP-A2-01 33 643.
All such methods do not provide a sufficiently durable protection for the metal pigment in the paint. It can be assumed that the additives, due to their poor adhesion to an aluminum surface, are peeled off or removed from the metal surface by organic solvents also present in aqueous paints, or by the amines and the paint binders. Furthermore, the hydrophobic character of the coating agents leads to wetting problems and possibly to a flocculation of the pigments unless sufficient quantities of emulsifiers are used. When emulsifiers are used, however, the resistance characteristics of the paint films are adversely influenced.
Another method to protect metal pigments from water consists in the coating of the pigment particles with crosslinked polymers. U.S. Pat. No. 3,616,397, for example, describes a method of coating the pigment particles with a thin coat of a crosslinked polymer by polymerization of acrylic monomers in the presence of the pigment. The polymerization is carried out in an organic solvent from which the coated pigment is separated by filtration. A disadvantage of this method is that several steps are necessary for the production of such special pigments, rendering the method uneconomical. Furthermore, there is a danger of damaging the very thin protective layer during the course of the production or the transport of the paint, i.e., with conventional cogwheel pumps in circular ducts, which will reduce the anticorrosive effect.
The same negative effects are realized according to the method disclosed by DE-AS No. 24 32 796 wherein the metal pigments are protected from atmospherical influences through a polymerization reaction with vinyl monomers. Although the adhesion of the protective coat to the previously activated surface of the pigment particles can be enhanced through the coemployment of phosphoric acid containing monomers, there still remains the disadvantage of the uneconomical production of the pigment preparations and the sensitivity of the protective layer to mechanical impact.
In order to avoid the aforesaid described disadvantages, AT-PS No. 372 696 corresponding to U.S. Pat. No. 4,453,982 discloses a method of stabilizing metal pigments from corroding media, particularly in anionic aqueous paints, by coating the pigment particles with a reaction product of an amino-formaldehyde condensate etherified with low molecular weight alcohols and phosphoric acid derivatives. However, it has been shown that paints containing the thus-coated metal pigments have to have higher pH-values to avoid flocculation, which in turn increases the aggressivity of the aqueous medium on the metal pigment to such an extent that the protective coating is ruptured and the aluminum pigment is slowly destroyed.
Another type of coating is described in DE-OS No. 30 20 073 corresponding to U.S. Pat. No. 4,419,134 according to which water-dispersible paste compositions of metal powders are obtained, if the coating materials are organic phosphoric acid esters of long-chain alcohols. These materials fixedly link with the metal, whereby the danger of separation or displacement by the binder is precluded.
EP-A1-01 33 644 corresponding to U.S. Pat. No. 4,565,716 discloses a similar method for stabilizing metal pigments by amine-neutralized phosphoric acid esters of alkyl phenols. Both such methods, however, have the disadvantage that in the aqueous medium the hydrophobic molecule segment leads to agglomeration of the pigment and, thus, to an unsightly metallic effect, unless corresponding quantities of emulsifiers are used in addition which in turn lead to other defects of the film.